There is an example of a conventional image forming apparatus in which an electrostatic capacitance detection apparatus detects a remaining amount of a toner in a toner container. For example, in a toner remaining amount detection apparatus described in PTL 1, a flexible member is coupled and fixed at one edge of an agitation member that agitates a toner in a toner container. A subject is fixed at a leading edge of the flexible member, and an electrostatic capacitance detection apparatus is provided at a lower part of the toner container. The flexible member coupled to the agitation member also rotates following the rotation of the agitation member and enters the toner. If the toner surface in the toner container is higher than the position where the flexible member and the agitation member are coupled, the flexible member enters the toner at the section coupled with the agitation member. The entire flexible member is flexibly transformed, and the flexible member rotates and moves at the same orbit (trajectory) as the coupled section in the toner. Therefore, the subject at the leading edge of the flexible member also rotates and moves at the same orbit as the flexible member. However, if the amount of the toner is reduced and the coupled section of the agitation member does not enter the toner when the toner surface is lower than the coupled position of the flexible member and the agitation member, the neighborhood of the leading edge of the flexible member slides over the toner surface, and the subject also slides and moves over the toner surface. If the remaining amount of the toner gradually decreases, the height of the toner surface in the toner container is also gradually reduced along with the decrease. The position of the subject that slides and moves over the toner surface is also gradually reduced. Therefore, if the toner decreases to less than a certain amount, the position of the subject that moves over the toner surface is also reduced according to the remaining amount of the toner, and the subject approaches the bottom of the toner container.
Meanwhile, the electrostatic capacitance detection apparatus can detect an electrostatic capacitance between the electrostatic capacitance detection apparatus and the subject that moves over the toner surface. The electrostatic capacitance between the electrostatic capacitance detection apparatus and the subject changes according to the distance between the two. The electrostatic capacitance detection apparatus is provided at a lower part of the toner container. Therefore, if the amount of the toner is reduced and the height of the toner surface is gradually lowered, the position of the subject on the toner surface is also lowered. As a result, the electrostatic capacitance between the two is reduced. Therefore, the electrostatic capacitance between the electrostatic capacitance detection apparatus and the subject changes according to the remaining amount of the toner.
In another conventional image forming apparatus, a permeability sensor is used as an apparatus that detects the amount of a toner in a developing unit. An example of the apparatus that uses the permeability sensor to detect the amount of the developer includes PTL 2. PTL 2 discloses a toner amount detection apparatus that uses a flexible first agitation blade that is transformed backward in a rotation direction based on agitation of the toner, a rigid second agitation blade provided backward in a rotation direction of the first agitation blade, and a permeability sensor provided outside of the bottom of the developing unit. In the apparatus, the permeability sensor provided outside of the bottom of the developing unit detects the state of rotational movements of metallic materials installed on the agitation blades. In the apparatus, the first agitation blade and the second agitation blade integrally make a rotational movement when the amount of the toner in the developing unit is large. The first agitation blade and the second agitation blade separately make rotational movements without transformation when the amount of the toner in the developing unit is small. In this case, when the permeability sensor is used for the detection, the change in the permeability per rotation of the rotation shaft is detected once if the amount of the toner in the developing unit is large. The change is detected twice if the amount of the toner in the developing unit is small. The toner amount detection apparatus detects the amount of the toner in the developing unit based on the change in the number of detections.